1. Field of the Invention
The present invention relates to semiconductor devices, and more specifically to decoupling between a noise emitter and a noise-sensitive receiver that are in an integrated circuit formed on a substrate that is suited for BiCMOS technology.
2. Description of Related Art
The coupling noise inside an integrated circuit comes essentially from two sources, namely the electromagnetic coupling due to the package and the metal lines of the circuit, and the electrical coupling due to the substrate. The present invention is concerned with reducing the electrical coupling noise due to the substrate. When a noise emitter injects charges into the substrate, they are distributed uniformly and can then be picked up by a receiver whose operation may be perturbed if it is sensitive to this electrical noise. For example, a "noise emitter" in the context of this description can be a strong-signal transistor that injects charges into the substrate, and the charges can be picked up by a weak-signal transistor that then behaves as a noise-sensitive receiver.
As an exemplary environment, such coupling noise will be described in relation to the field of mobile telephones. After a radio frequency reception stage, a mobile telephone has a low-noise amplifier with transistors for processing signals of very low power, typically on the order of 1 nanowatt to 100 nanowatts. This low-noise amplifier is followed by a mixer for carrying out frequency transposition from a signal output by a voltage-controlled oscillator associated with a phase-locked loop, which receives a signal output by a local oscillator and has a frequency divider.
Typically, the voltage-controlled oscillator processes signals on the order of 100 mvolts while the phase-locked loop processes signals on the order of 200 mvolts and has (in particular for the digital portion) CMOS transistors powered at 3 volts. These signals typically have a power on the order of one watt, which is high when compared with the power of the signals processed by the low-noise amplifier. Therefore, in the substrate, there is high-frequency analog noise associated with the voltage-controlled oscillator, as well as wide-band digital noise associated with the phase-locked loop. The combination of these perturbations creates white noise whose power in decibels is liable to perturb the low-noise amplifier if all of these components are produced within the same integrated circuit.
One current solution for overcoming such problems is to produce the low-noise amplifier, the voltage-controlled oscillator, the phase-locked loop, and the mixer on separate integrated circuits. For example, the low-noise amplifier and the voltage-controlled oscillator can be produced on two separate circuits having gallium arsenide substrates, while the phase-locked loop and the mixer can be produced on a silicon integrated circuit, and more particularly one appropriate to BiCMOS technology (i.e., bipolar CMOS technology in which bipolar transistors are combined with insulated-gate complementary field-effect transistors). However, this solution causes an increase in the overall area of the device overall, and in its cost.